The use of local area networks (LANs) to communicate messages among computers is well known. One of the standard protocols for controlling access to a LAN is the ANSI/IEEE 802.3 carrier-sense-multiple-access with collision detection (CSMA/CD) Standard, commonly known as the EtherNet.RTM. standard.
In a CSMA/CD protocol LAN, all of the nodes or stations on the network are connected to a common carrier, such as a coaxial cable. When a node is ready to transmit a message, it monitors the common media to determine whether any other transmissions are occurring. If the node senses that the media is busy (i.e., a carrier signal is present), transmission of the message is deferred until the media is not busy. When the node senses that the media is not busy, transmission of the message is attempted. Due to the fact that multiple nodes that are separated by finite physical distances along the common media may simultaneously attempt to transmit a message, it is possible that overlapping messages can be transmitted, resulting in a collision of different messages. In this case, each node stops transmitting and waits some random period of time before reattempting transmission of the message.
It will be recognized that for any given node on the LAN, a CSMA/CD protocol provides for an indeterminate, random-type of contention algorithm that controls access to the common media. In other words, under a standard CSMA/CD protocol, a node cannot be guaranteed that the media will be available for transmission when that node has a message ready to transmit. For typical computer-to-computer transmissions across a LAN, the indeterminate nature of the CSMA/CD contention algorithm works quite well due to the sporadic or bursty nature of most computer transmissions. Much of the effort in improving CSMA/CD protocol LANs has focused on evaluating and optimizing the effective throughput of the LAN based on an analysis of the probability of obtaining access given the indeterminate nature of the contention algorithm. Tobagi, F. and Hunt, B., "Performance Analysis of Carrier Sense Multiple Access with Collision Detection", Advances in Local Area Networks, IEEE Press (1987), Chpt. 20, pp. 318-339.
Although it is possible to optimize the indeterminate nature of the contention algorithm for a CSMA/CD protocol LAN for typical computer transmissions, the CSMA/CD type of indeterminate contention algorithms are, almost by definition, not well suited to implementing the transmission of real time digital video information or other types of digital transmissions that require a known or deterministic access capacity across the LAN. In the case of a terminal running a video simulation application where the data for the simulation is stored on a network server, that terminal may require an effective and sustained data transfer rate across the LAN. In addition, because messages transmitted across a typical CSMA/CD protocol LAN are limited to a predetermined maximum packet length, such as 1500 eight bit bytes for the EtherNet.RTM. protocol, the node running the video simulation application will be forced to give up control of the LAN even if it is able to acquire access to the LAN in the first place.
Some of the newer LAN protocols, such as the ATM protocol or the communication protocols described in U.S. Pat. Nos. 4,787,082, 5,130,983 and 5,175,732, can allocate a fixed amount of network capacity to a given node so as to support the transmission of real time digital video information, for example. While many of these new types of LAN protocols can effectively support the type of deterministic access to the LAN that is required, they necessarily require the installation of a completely new network. Due to the tremendous investment in existing LANs, it would be desirable to allow the existing base of LANs to provide for deterministic access to the network.
Unfortunately, all of the current solutions to this problem for existing CSMA/CD protocol LANs have relied on changing the hardware and/or the software on all of the nodes in the CSMA/CD protocol LAN in order to allow for deterministic access to the LAN, as shown for example in U.S. Pat. Nos. 4,750,171 and 4,819,229. The addition of this type of hardware and/or software to every single node in the LAN is also a costly solution when only one or two nodes on the LAN may actually utilize real time digital video displays that require a deterministic data transfer rate across the LAN.
Accordingly, it would be desirable to solve the problem of providing a deterministic allocation of capacity for existing CSMA/CD protocol LANs, without requiring that additional software and/or hardware be added to user nodes in the network. In addition, it would be desirable to provide a deterministic access contention algorithm for a CSMA/CD protocol LAN that has the flexibility to utilize the standard indeterminate contention algorithm for a CSMA/CD protocol for those user nodes that do not require deterministic access to the network.